Guard entrypoint() against OSError from an unreadable .env

[woutervh]

Summary

get_dotenv_path() only checks is_file() before entrypoint() hands the path to
dotenv.load_dotenv(). A permission-denied (or otherwise unreadable) .env passes that
check but fails to open, and the resulting OSError was previously uncaught - it
propagates out of entrypoint() and crashes every Python process started in the venv,
including recovery commands like pip/uv.

Changes

• src/autoread_dotenv/__init__.py: added an except OSError as exc: branch around
  dotenv.load_dotenv(...) that warns via simple_warning() instead of crashing.
• While in there: fixed the existing "django-dotenv installed instead of python-dotenv"
  AttributeError branch to also go through simple_warning() (it bypassed it before,
  inconsistent with the other two failure paths in the function), and cleaned up a
  stray-whitespace bug from a line-continuation in that message.
• docs/changes.md: logged under ## Unreleased.
• tests/test_autoread.py: new chmod(0o000)-based test asserting entrypoint() warns
  instead of raising, and that nothing gets loaded into os.environ. Skipped on Windows,
  where permission-denial via chmod isn't reliable.

Testing

• pytest: 19 passed, 100% coverage (>= 90% required) - the new OSError branch is
  genuinely exercised, not pragma: no coverd.
• ruff check .: clean
• ty check: clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a8f5708) to head (4065173).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #125   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           66        69    +3     
  Branches         3         3           
=========================================
+ Hits            66        69    +3     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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